Copolymers of aliphatic monoethylenically unsaturated mono- and dicarboxylic acids are well known and have been used as incrustation inhibitors and deflocculants in aqueous systems, and as builders in detergent compositions. Such copolymers have been prepared by copolymerizing a monoethylenically unsaturated aliphatic dicarboxylic acid, such as maleic acid, with one or more monoethylenically unsaturated monocarboxylic acids, and/or monoethylenically unsaturated sulfonic acids. Examples of monocarboxylic acid monomers are acrylic and methacrylic acid. These acid monomers are collectively referred to in the specification and appended claims as "(meth)acrylic acid".
Various processes for preparing such copolymers in an aqueous solvent system have been described in the literature. Thus, U.S. Pat. Nos. 3,806,367 and 3,898,037 disclose, inter alia, the preparation of copolymers of the disodium salt of maleic acid and acrylamido-sulfonic cid compounds, especially monomeric 2-acrylamido-2-methylpropane sulfonic acid in an aqueous system in the presence of an organic chain transfer agent such as isopropanol. Although the amount of residual maleic acid monomer is not stated, it is believed that the degree of conversion of monomer to copolymer is on the order of about 80 percent. In addition, the solids content of the resulting copolymer solution is on the order of 15 to 25 percent.
U.S. Pat. No. 4,314,044 discloses preparation of low molecular weight water soluble copolymers of acrylic and methacrylic acid with monoethylenically unsaturated dicarboxylic acids such as itaconic, fumaric and maleic acid, in an aqueous solvent system containing an initiator and a metal salt in which the molar ratio of initiator to metal ion of the salt is from 10:1 to 150:1, and the initiator is present in an amount of from 0.5 weight percent to 35 weight percent based on the weight of monomers present. Depending upon the monomers used, the degree of monomer conversion varies from about 75 to about 95 percent. Thus, the copolymer solution contains substantial amounts of unreacted monomer. The problem of unreacted residual monomer would be even greater with copolymers of maleic acid, a less reactive monomer.
Published German patent application No. P 31 38 574.5 discloses a process for the production of copolymers of monoethylenically unsaturated monocarboxylic acids with dicarboxylic acids in aqueous medium in the presence of a water soluble radical generating initiator, such as a peroxide, hydroperoxide or persulfate. In such process, to an aqueous solution of the partially neutralized salt of the dicarboxylic acid monomer there are added over a period of hours an aqueous solution of an .alpha.,.beta.-ethylenically unsaturated monocarboxylic acid monomer and an initiator, while the temperature of the reaction medium is maintained at about 60.degree. C. to 150.degree. C. During the polymerization 20% to 80% of the carboxyl groups of the monomers are neutralized. However, such process requires the use of large amounts of initiator and relatively high polymerization temperatures to obtain a copolymer solution containing a relatively low residual content of unpolymerized dicarboxylic acid monomer. The presence of dicarboxylic acid monomer in the aqueous polymer solution decreases the effectiveness of the copolymer in many uses, an example of which is as an encrustation inhibitor. In addition, the dicarboxylic acid monomer, e.g. maleic acid, physiologically is not a totally harmless substance.
In an effort to reduce the amount of residual unpolymerized dicarboxylic acid monomer it has been suggested to carry out the process of German Application No. P 31 47 489 in the presence of a water soluble radical generating initiator comprising a combination of hydrogen peroxide and a persulfate, the weight ratio of the former to the latter being from 3:1 to 1:3. See published European Application No. 01 06 110, the disclosure of which and that of the above-discussed German Application No. P 31 47 489 are incorporated herein by reference. Although such combination of initiators significantly reduces the residual content of unpolymerized dicarboxylic acid, the use of such process makes it difficult to obtain relatively high molecular weight copolymers while at the same time maintaining a high degree of monomer conversion.